The thyristors of polyphase thyristor-pulse converters are controlled by time-shifted and time-constant trains of control pulses applied to the corresponding thyristors of this converter. In this case, in order to reduce the current fluctuation in the power supply source and in the load, it is necessary that, simultaneously with the process of shaping the time-shifted and time-constant pulse trains, the start of operation of each successive phase of the polyphase thyristor-pulse converter is shifted with respect to preceding phase by (1/m) T, where m is the number of phases of the thyristor-pulse converter, T is the switching period of the thyristor-pulse converter.
Well known in the art is a device for digital control of a polyphase thyristor-pulse converter (cf. "Proceedings of the Dnepropetrovsk Institute of Railway Engineers" vol. 106, 1971, pp. 65-70). This device includes a clock counter and a bidirectional counter connected to a decoder, a switch and a clock pulse generator. In this device the filling factor, which is the ratio of the duration of the conductive state of the thyristor of the thyristor-pulse converter to the period of switching the thyristors of the thyristor-pulse converter, is varied digitally. The discretity value depends on the number of triggers in the counters.
However, this device comprises a great number of elements and this results in low operational reliability, a large size and high cost.
Also known in the art is a device for digital control of thyristor-pulse converters (cf. USSR Inventor's Certificate No. 424290, 1971), which includes a master oscillator connected to the input of a clock pulse counter unit having a plurality of clock counters whose number correponds to that of the phases.
The device also includes several decoders whose number is equal to that of the phases of the thyristor-pulse converter. The logical outputs of the clock pulse counter unit are connected to the logical inputs of every decoder. In this case each decoder is connected only to the logical outputs of one clock pulse counter of the clock pulse counter unit. Furthermore, the device has a unit for counting the control pulses, a plurality of bidirectional pulse counters, the number of which is equal to the number of phases, and which are connected to the other logical inputs of the decoders. Thus, in the known device it is necessary to use several bidirectional pulse counters of the control pulse counter unit and several clock pulse counters of the clock pulse counter unit for shaping a plurality of time-shifted and time-constant pulse trains, the number of said counters being equal to the number of phases of the thyristor-pulse converter. This reduces the reliability, increases the input power, size, cost and also deteriorates the quality of control due to the technological scatter of the parameters of the employed pulse counters. The scatter of parameters of the pulse counters also affect the shift and time position of the pulses thus changing the power current and worsening the spectrum of harmonic components in the power source.